This invention relates to a process control instrument.
More particularly this invention relates to a process control instrument for controlling a process variable during a batch reaction cycle.
Even more particularly this invention relates to an integral limiting device for a process controller which will eliminate overshoot of the controlled process variable during an exothermic batch reaction cycle.
Heretofore it has been a common practice to install an anti-reset windup module in instruments that are removed from control of the process periodically, for example in a batch process. Without the anti-reset windup module, whenever a sustained deviation from the setpoint is imposed upon a control instrument, the instrument output will eventually be driven off scale. If the instrument were then returned to control of the process, no control action could be taken by the instrument until it has recovered from being driven off scale, that is until after the setpoint has been crossed by the process variable. The anti-reset windup is used to prevent the instrument from being driven off scale. However, even though the use of the anti-reset windup module will reduce the overshoot, that is the amount by which the process variable exceeds the setpoint, it will not eliminate the overshoot entirely unless the module's high limit is set so low as to greatly extend the time required to complete a batch cycle. Thus for a batch process the present anti-reset windup module is useful but not completely successful.